Material characterization of in vivo and in vitro porcine brain using shear wave elasticity

Realistic computer simulation of closed head trauma requires accurate mechanical properties of brain tissue, ideally in vivo. A substantive deficiency of most existing experimental brain data is that properties were identified through in vitro mechanical testing. This study develops a novel application of shear wave elasticity imaging to assess porcine brain tissue shear modulus in vivo. Shear wave elasticity imaging is a quantitative ultrasound technique that has been used here to examine changes in brain tissue shear modulus as a function of several experimental and physiologic parameters. Animal studies were performed using two different ultrasound transducers to explore the differences in physical response between closed skull and open skull arrangements. In vivo intracranial pressure in four animals was varied over a relevant physiologic range (2-40 mmHg) and was correlated with shear wave speed and stiffness estimates in brain tissue. We found that stiffness does not vary with modulation of intracranial pressure. Additional in vitro porcine specimens (n = 14) were used to investigate variation in brain tissue stiffness with temperature, confinement, spatial location and transducer orientation. We observed a statistically significant decrease in stiffness with increased temperature (23%) and an increase in stiffness with decreasing external confinement (22-37%). This study determined the feasibility of using shear wave elasticity imaging to characterize porcine brain tissue both in vitro and in vivo. Our results underline the importance of temperature- and skull-derived boundary conditions to brain stiffness and suggest that physiologic ranges of intracranial pressure do not significantly affect in situ brain tissue properties. Shear wave elasticity imaging allowed for brain material properties to be experimentally characterized in a physiologic setting and provides a stronger basis for assessing brain injury in computational models.

Significant head accelerations can influence immediate neurological impairments in a murine model of blast-induced traumatic brain injury

Although blast-induced traumatic brain injury (bTBI) is well recognized for its significance in the military population, the unique mechanisms of primary bTBI remain undefined. Animate models of primary bTBI are critical for determining these potentially unique mechanisms, but the biomechanical characteristics of many bTBI models are poorly understood. In this study, we examine some common shock tube configurations used to study blast-induced brain injury in the laboratory and define the optimal configuration to minimize the effect of torso overpressure and blast-induced head accelerations. Pressure transducers indicated that a customized animal holder successfully reduced peak torso overpressures to safe levels across all tested configurations. However, high speed video imaging acquired during the blast showed significant head accelerations occurred when animals were oriented perpendicular to the shock tube axis. These findings of complex head motions during blast are similar to previous reports [Goldstein et al., 2012, "Chronic Traumatic Encephalopathy in Blast-Exposed Military Veterans and a Blast Neurotrauma Mouse Model," Sci. Transl. Med., 4(134), 134ra160; Sundaramurthy et al., 2012, "Blast-Induced Biomechanical Loading of the Rat: An Experimental and Anatomically Accurate Computational Blast Injury Model," J. Neurotrauma, 29(13), pp. 2352-2364; Svetlov et al., 2010, "Morphologic and Biochemical Characterization of Brain Injury in a Model of Controlled Blast Overpressure Exposure," J. Trauma, 69(4), pp. 795-804]. Under the same blast input conditions, minimizing head acceleration led to a corresponding elimination of righting time deficits. However, we could still achieve righting time deficits under minimal acceleration conditions by significantly increasing the peak blast overpressure. Together, these data show the importance of characterizing the effect of blast overpressure on head kinematics, with the goal of producing models focused on understanding the effects of blast overpressure on the brain without the complicating factor of superimposed head accelerations.

Injury Risk in Behind Armor Blunt Thoracic Trauma

First responders and military personnel are particularly susceptible to behind armor blunt thoracic trauma in occupational scenarios. The objective of this study was to develop an armored thorax injury risk criterion for short duration ballistic impacts. 9 cadavers and 2 anthropomorphic test dummies (AUSMAN and NIJ 0101.04 surrogate) were tested over a range of velocities encompassing low severity impacts, medium severity impacts, and high severity impacts based upon risk of sternal fracture. Thoracic injuries ranged from minor skin abrasions (abbreviated injury scale [AIS] 1) to severe sternal fractures (AIS 3+) and were well correlated with impact velocity and bone mineral density. 8 male cadavers were used in the injury risk criterion development. A 50% risk of AIS 3+ injury corresponded to a peak impact force of 24,900 +/- 1,400 N. The AUSMAN impact force correlated strongly with impact velocity. Recommendations to improve the biofidelity of the AUSMAN include implementing more realistic viscera and decreasing the skin thickness.

Three-dimensional adult male head and skull contours

OBJECTIVE

Traumatic brain injury (TBI) is a major public health issue, affecting millions of people annually. Anthropomorphic test devices (ATDs) and finite element models (FEMs) provide a means of understanding factors leading to TBI, potentially reducing the occurrence. Thus, there is a need to ensure that these tools accurately model humans. For example, the Hybrid III was not based on 3-dimensional human head shape data. The objective of this study is to produce average head and skull contours for an average U.S. male that can be used for ATDs and FEMs.

METHODS

Computed tomography (CT) scans of adult male heads were obtained from a database provided by the University of Virginia Center for Applied Biomechanics. An orthographic viewer was used to extract head and skull contours from the CT scans. Landmarks were measured graphically using HyperMesh (Altair, HyperWorks). To determine the head occipital condyle (OC) centroid, surface meshes of the OCs were made and the centroid of the surfaces was calculated. The Hybrid III contour was obtained using a MicroScribe Digitizer (Solution Technologies, Inc., Oella, MD). Comparisons of the average male and ATD contours were performed using 2 methods: (1) the midsagittal and midcoronal ATD contours relative to the OC centroid were compared to the corresponding 1 SD range of the average male contours; (2) the ATD sagittal contour was translated relative to the average male sagittal contour to minimize the area between the 2 contours.

RESULTS

Average male head and skull contours were created. Landmark measurements were made for the dorsum sellae, nasion skin, nasion bone, infraorbital foramen, and external auditory meatus, all relative to the OC centroid. The Hybrid III midsagittal contour was outside the 1 SD range for 15.2 percent of the average male head contour but only by a maximum distance of 1.5 mm, whereas the Hybrid III midcoronal head contour was outside the 1 SD range for 12.2 percent of the average male head contour by a maximum distance of 2 mm. Minimization of the area between the midsagittal contours resulted in only 2.3 mm of translation, corroborating the good correlation between the contours established by initial comparison.

CONCLUSIONS

Three-dimensional average male head and skull contours were created and measurements of landmark locations were made. It was found that the 50th percentile male Hybrid III corresponds well to the average male head contour and validated its 3D shape. Average adult head and skull contours and landmark data are available for public research use at http://biomechanics.pratt.duke.edu/data .

Pediatric head and neck dynamics in frontal impact: analysis of important mechanical factors and proposed neck performance corridors for 6- and 10-year-old ATDs

OBJECTIVE

Traumatic injuries are the leading cause of death of children aged 1-19 in the United States and are principally caused by motor vehicle collisions, with the head being the primary region injured. The neck, though not commonly injured, governs head kinematics and thus influences head injury. Vehicle improvements necessary to reduce these injuries are evaluated using anthropomorphic testing devices (ATDs). Current pediatric ATD head and neck properties were established by scaling adult properties using the size differences between adults and children. Due to the limitations of pediatric biomechanical research, computational models are the only available methods that combine all existing data to produce injury-relevant biofidelity specifications for ATDs. The purpose of this study is to provide the first frontal impact biofidelity corridors for neck flexion response of 6- and 10-year-olds using validated computational models, which are compared to the Hybrid III (HIII) ATD neck responses and the Mertz flexion corridors.

METHODS

Our virtual 6- and 10-year-old head and neck multibody models incorporate pediatric biomechanical properties obtained from pediatric cadaveric and radiological studies, include the effect of passive and active musculature, and are validated with data including pediatric volunteer 3 g dynamic frontal impact responses. We simulate ATD pendulum tests-used to calibrate HIII neck bending stiffness-to compare the pediatric model and HIII ATD neck bending stiffness and to compare the model flexion bending responses with the Mertz scaled neck flexion corridors. Additionally, pediatric response corridors for pendulum calibration tests and high-speed (15 g) frontal impacts are estimated through uncertainty analyses on primary model variables, with response corridors calculated from the average ± SD response over 650 simulations.

RESULTS AND CONCLUSIONS

The models are less stiff in dynamic anterioposterior bending than the ATDs; the secant stiffness of the 6- and 10-year-old models is 53 and 67 percent less than that of the HIII ATDs. The ATDs exhibit nonlinear stiffening and the models demonstrate nonlinear softening. Consequently, the models do not remain within the Mertz scaled flexion bending corridors. The more compliant model necks suggest an increased potential for head impact via larger head excursions. The pediatric anterioposterior bending corridors developed in this study are extensible to any frontal loading condition through calculation and sensitivity analysis. The corridors presented in this study are the first based on pediatric cadaveric data and provide the basis for future, more biofidelic, designs of 6- and 10-year-old ATD necks.

Importance of muscle activations for biofidelic pediatric neck response in computational models

OBJECTIVE

During dynamic injury scenarios, such as motor vehicle crashes, neck biomechanics contribute to head excursion and acceleration, influencing head injuries. One important tool in understanding head and neck dynamics is computational modeling. However, realistic and stable muscle activations for major muscles are required to realize meaningful kinematic responses. The objective was to determine cervical muscle activation states for 6-year-old, 10-year-old, and adult 50th percentile male computational head and neck models. Currently, pediatric models including muscle activations are unable to maintain the head in an equilibrium position, forcing models to begin from nonphysiologic conditions. Recent work has realized a stationary initial geometry and cervical muscle activations by first optimizing responses against gravity. Accordingly, our goal was to apply these methods to Duke University's head-neck model validated using living muscle response and pediatric cadaveric data.

METHODS

Activation schemes maintaining an upright, stable head for 22 muscle pairs were found using LS-OPT. Two optimization problems were investigated: a relaxed state, which minimized muscle fatigue, and a tensed activation state, which maximized total muscle force. The model's biofidelity was evaluated by the kinematic response to gravitational and frontal impact loading conditions. Model sensitivity and uncertainty analyses were performed to assess important parameters for pediatric muscle response. Sensitivity analysis was conducted using multiple activation time histories. These included constant activations and an optimal muscle activation time history, which varied the activation level of flexor and extensor groups, and activation initiation and termination times.

RESULTS

Relaxed muscle activations decreased with increasing age, maintaining upright posture primarily through extensor activation. Tensed musculature maintained upright posture through coactivation of flexors and extensors, producing up to 32 times the force of the relaxed state. Without muscle activation, the models fell into flexion due to gravitational loading. Relaxed musculature produced 28.6-35.8 N of force to the head, whereas tensed musculature produced 450-1023 N. Pediatric model stiffnesses were most sensitive to muscle physiological cross-sectional area.

CONCLUSIONS

Though muscular loads were not large enough to cause vertebral compressive failure, they would provide a prestressed state that could protect the vertebrae during tensile loading but might exacerbate risk during compressive loading. For example, in the 10-year-old, a load of 602 N was produced, though estimated compressive failure tolerance is only 2.8 kN. Including muscles and time-variant activation schemes is vital for producing biofidelic models because both vary by age. The pediatric activations developed represent physiologically appropriate sets of initial conditions and are based on validated adult cadaveric data.

How few? Bayesian statistics in injury biomechanics

In injury biomechanics, there are currently no general a priori estimates of how few specimens are necessary to obtain sufficiently accurate injury risk curves for a given underlying distribution. Further, several methods are available for constructing these curves, and recent methods include Bayesian survival analysis. This study used statistical simulations to evaluate the fidelity of different injury risk methods using limited sample sizes across four different underlying distributions. Five risk curve techniques were evaluated, including Bayesian techniques. For the Bayesian analyses, various prior distributions were assessed, each incorporating more accurate information. Simulated subject injury and biomechanical input values were randomly sampled from each underlying distribution, and injury status was determined by comparing these values. Injury risk curves were developed for this data using each technique for various small sample sizes; for each, analyses on 2000 simulated data sets were performed. Resulting median predicted risk values and confidence intervals were compared with the underlying distributions. Across conditions, the standard and Bayesian survival analyses better represented the underlying distributions included in this study, especially for extreme (1, 10, and 90%) risk. This study demonstrates that the value of the Bayesian analysis is the use of informed priors. As the mean of the prior approaches the actual value, the sample size necessary for good reproduction of the underlying distribution with small confidence intervals can be as small as 2. This study provides estimates of confidence intervals and number of samples to allow the selection of the most appropriate sample sizes given known information.

Attenuation of blast pressure behind ballistic protective vests

BACKGROUND

Clinical studies increasingly report brain injury and not pulmonary injury following blast exposures, despite the increased frequency of exposure to explosive devices. The goal of this study was to determine the effect of personal body armour use on the potential for primary blast injury and to determine the risk of brain and pulmonary injury following a blast and its impact on the clinical care of patients with a history of blast exposure.

METHODS

A shock tube was used to generate blast overpressures on soft ballistic protective vests (NIJ Level-2) and hard protective vests (NIJ Level-4) while overpressure was recorded behind the vest.

RESULTS

Both types of vest were found to significantly decrease pulmonary injury risk following a blast for a wide range of conditions. At the highest tested blast overpressure, the soft vest decreased the behind armour overpressure by a factor of 14.2, and the hard vest decreased behind armour overpressure by a factor of 56.8. Addition of body armour increased the 50th percentile pulmonary death tolerance of both vests to higher levels than the 50th percentile for brain injury.

CONCLUSIONS

These results suggest that ballistic protective body armour vests, especially hard body armour plates, provide substantial chest protection in primary blasts and explain the increased frequency of head injuries, without the presence of pulmonary injuries, in protected subjects reporting a history of blast exposure. These results suggest increased clinical suspicion for mild to severe brain injury is warranted in persons wearing body armour exposed to a blast with or without pulmonary injury.

A Multiscale Approach to Blast Neurotrauma Modeling: Part I - Development of Novel Test Devices for in vivo and in vitro Blast Injury Models

The loading conditions used in some current in vivo and in vitro blast-induced neurotrauma models may not be representative of real-world blast conditions. To address these limitations, we developed a compressed-gas driven shock tube with different driven lengths that can generate Friedlander-type blasts. The shock tube can generate overpressures up to 650 kPa with durations between 0.3 and 1.1 ms using compressed helium driver gas, and peak overpressures up to 450 kPa with durations between 0.6 and 3 ms using compressed nitrogen. This device is used for short-duration blast overpressure loading for small animal in vivo injury models, and contrasts the more frequently used long duration/high impulse blast overpressures in the literature. We also developed a new apparatus that is used with the shock tube to recreate the in vivo intracranial overpressure response for loading in vitro culture preparations. The receiver device surrounds the culture with materials of similar impedance to facilitate the propagation of a single overpressure pulse through the tissue. This method prevents pressure waves reflecting off the tissue that can cause unrealistic deformation and injury. The receiver performance was characterized using the longest helium-driven shock tube, and produced in-fluid overpressures up to 1500 kPa at the location where a culture would be placed. This response was well correlated with the overpressure conditions from the shock tube (R² = 0.97). Finite element models of the shock tube and receiver were developed and validated to better elucidate the mechanics of this methodology. A demonstration exposing a culture to the loading conditions created by this system suggest tissue strains less than 5% for all pressure levels simulated, which was well below functional deficit thresholds for strain rates less than 50 s⁻¹. This novel system is not limited to a specific type of culture model and can be modified to reproduce more complex pressure pulses.

A Multiscale Approach to Blast Neurotrauma Modeling: Part II: Methodology for Inducing Blast Injury to in vitro Models

Due to the prominent role of improvised explosive devices (IEDs) in wounding patterns of U.S. war-fighters in Iraq and Afghanistan, blast injury has risen to a new level of importance and is recognized to be a major cause of injuries to the brain. However, an injury risk-function for microscopic, macroscopic, behavioral, and neurological deficits has yet to be defined. While operational blast injuries can be very complex and thus difficult to analyze, a simplified blast injury model would facilitate studies correlating biological outcomes with blast biomechanics to define tolerance criteria. Blast-induced traumatic brain injury (bTBI) results from the translation of a shock wave in-air, such as that produced by an IED, into a pressure wave within the skull-brain complex. Our blast injury methodology recapitulates this phenomenon in vitro, allowing for control of the injury biomechanics via a compressed-gas shock tube used in conjunction with a custom-designed, fluid-filled receiver that contains the living culture. The receiver converts the air shock wave into a fast-rising pressure transient with minimal reflections, mimicking the intracranial pressure history in blast. We have developed an organotypic hippocampal slice culture model that exhibits cell death when exposed to a 530 ± 17.7-kPa peak overpressure with a 1.026 ± 0.017-ms duration and 190 ± 10.7 kPa-ms impulse in-air. We have also injured a simplified in vitro model of the blood-brain barrier, which exhibits disrupted integrity immediately following exposure to 581 ± 10.0 kPa peak overpressure with a 1.067 ± 0.006-ms duration and 222 ± 6.9 kPa-ms impulse in-air. To better prevent and treat bTBI, both the initiating biomechanics and the ensuing pathobiology must be understood in greater detail. A well-characterized, in vitro model of bTBI, in conjunction with animal models, will be a powerful tool for developing strategies to mitigate the risks of bTBI.

Pediatric head contours and inertial properties for ATD design

Child head trauma in the United States is responsible for 30% of all childhood injury deaths with costs estimated at $10 billion per year. The common tools for studying this problem are the child anthropomorphic test devices (ATDs). The headform sizes and structural properties of child ATDs are based on various anthropometric studies and scaled Hybrid III mass and center of gravity (CG) properties. The goals of this study were to produce pediatric head and skull contours, provide estimates of pediatric head mass, mass moment of inertia and CG locations, and compare the head contours with the current child ATD head designs. To that end, computer tomography (CT) scans from one hundred eighty-five children in twelve age groups were analyzed to develop three-dimensional head and skull contours. The contours were averaged to estimate head and skull contours for children aged 1-month to 10-years. Inertial properties were estimated from a small sample of post- mortem human subjects (PMHSs). This paper provides new equations for estimating the moments of inertia and anatomical landmarks in the head. There were reasonable agreement between the estimates for head masses obtained from analysis of the CT scans of the PMHS heads and the estimates obtained using the volumetric scaling rule used in ATD design work. The regression of the pediatric head sizes was found to be non-linear, with different regression slope for ages 1M to 18M and 18M to 120M. The 12M CRABI and 36M Hybrid III heads were found to be different by 10 and 18mm, respectively, from the average human CT contours due to the differences in the occipital condyle placement relative to the nasion.

Drosophila melanogaster larvae as a model for blast lung injury

BACKGROUND

Primary blast injuries, specifically lung injuries, resulting from blast overpressure exposures are a major source of mortality for victims of blast events. However, existing pulmonary injury criteria are inappropriate for common exposure environments. This study uses Drosophila melanogaster larvae to develop a simple phenomenological model for human pulmonary injury from primary blast exposure.

METHODS

Drosophila larvae were exposed to blast overpressures generated by a 5.1-cm internal diameter shock tube and their mortality was observed after the exposure. To establish mortality thresholds, a survival analysis was conducted using survival data and peak incident pressures. In addition, a histologic analysis was performed on the larvae to establish the mechanisms of blast injury.

RESULTS

The results of the survival analysis suggest that blast overpressure for 50% Drosophila survival is greater than human threshold lung injury and is similar to human 50% survival levels, in the range of overpressure durations tested (1-5 ms). A "parallel" analysis of the Bass et al. 50% human survival curves indicates that 50% Drosophila survival is equivalent to a human injury resulting in a 69% chance of survival. Histologic analysis of the blast-exposed larvae failed to demonstrate damage to the dorsal trunk of the tracheal system; however, the presence of flocculent material in the larvae body cavities and tracheas suggests tissue damage.

CONCLUSIONS

This study shows that D. melanogaster survival can be correlated with large animal injury models to approximate a human blast lung injury tolerance. Within the range of durations tested, Drosophila larvae may be used as a simple model for blast injury.

Comparison of Hybrid III child test dummies to pediatric PMHS in blunt thoracic impact response

The limited availability of pediatric biomechanical impact response data presents a significant challenge to the development of child dummies. In the absence of these data, the development of the current generation of child dummies has been driven by scaling of the biomechanical response requirements of the existing adult test dummies. Recently published pediatric blunt thoracic impact response data provide a unique opportunity to evaluate the efficacy of these scaling methodologies. However, the published data include several processing anomalies and nonphysical features. These features are corrected by minimizing instrumentation and processing error to improve the fidelity of the individual force-deflection responses. Using these data, biomechanical impact response corridors are calculated for a 3-year-old child and a 6-year-old child. These calculated corridors differ from both the originally published postmortem human subject (PMHS) corridors and the impact response requirements of the current child dummies. Furthermore, the response of the Hybrid III 3-year-old test dummy in the same impact condition shows a similar deflection but a significantly higher force than the 3-year-old corridor. The response of the Hybrid III 6-year-old dummy, on the other hand, correlates well with the calculated 6-year-old corridor. The newly developed 3-year-old and 6-year-old blunt thoracic impact response corridors can be used to define data-driven impact response requirements as an alternative to scaling-driven requirements.

Kinematic response of the spine during simulated aircraft ejections

INTRODUCTION

Military aviators are susceptible to spinal injuries during high-speed ejection scenarios. These injuries commonly arise as a result of strains induced by extreme flexion or compression of the spinal column. This study characterizes the vertebral motion of two postmortem human surrogates (PMHS) during a simulated catapult phase of ejection on a horizontal decelerator sled.

METHODS

During testing, the PMHS were restrained supinely to a mock ejection seat and subjected to a horizontal deceleration profile directed along the local z-axis. Two midsized males (175.3 cm, 77.1 kg; 185.4 cm, 72.6 kg) were tested. High-rate motion capture equipment was used to measure the three-dimensional displacement of the head, vertebrae, and pelvis during the ejection event.

RESULTS

The two PMHS showed generally similar kinematic motion. Head injury criterion (HIC) results were well below injury threshold levels for both specimens. The specimens both showed compression of the spine, with a reduction in length of 23.9 mm and 45.7 mm. Post-test autopsies revealed fractures in the C5, T1, and L1 vertebrae.

DISCUSSION

This paper provides an analysis of spinal motion during an aircraft ejection.The injuries observed in the test subjects were consistent with those seen in epidemiological studies. Future studies should examine the effects of gender, muscle tensing, out-of-position (of head from neutral position) occupants, and external forces (e.g., windblast) on spinal kinematics during aircraft ejection.

Re-evaluating the neck injury index (NII) using experimental PMHS tests

OBJECTIVE

The neck injury index, NII, developed in ISO 13232 (2005) as a testing and evaluation procedure for assessing the risk of injury to the AO/C1/C2 region of the cervical spine in motorcycle riders is reevaluated using an existing postmortem human subjects (PMHS) data set and resulting in a reformulated NII criterion applicable to PMHS tests.

METHODS

A recent series of 36 PMHS head/neck component tests was used to examine the risk of neck injury in frontal impacts and to assess the predictive capability of NII for impacts of various orientations. Using force and moment load cell PMHS experimental data, injury risk was assessed using NII evaluated with the ISO 13232-5 algorithms.

RESULTS

The injury risk predictions are compared with the injury outcomes from the head/neck PMHS. The NII criterion underestimated the injury incidence of the PMHS experimental group. The average predicted risk of injuries for the experimental injury tests based on NII across the MAIS levels was 0.7 percent, though there were 11 AIS 3+ injuries observed in the actual testing (30.6%). Using the experimental injury outcomes and the experimental force and moment time histories, the normalizing coefficients from NII are reevaluated to minimize the difference between NII risk assessment and the experimental injury outcome in the least squares (L(2)) basis. This reanalysis is compared with existing human and PMHS neck injury criteria.

CONCLUSIONS

By reanalyzing the NII formulation using an existing PMHS injury data set with known forces and moments and known injury outcomes, a new NII(PMHS) is developed that uses PMHS loads to predict injury. This reformulation removes the dependency of the original NII formulation on the forces and moments from motorcyclist anthropomorphic test device (MATD) experiments and simulations yet retains the advantages of the multi-axial neck injury criterion.

Pediatric thoracoabdominal biomechanics

No experimental data exist quantifying the force-deformation behavior of the pediatric chest when subjected to non-impact, dynamic loading from a diagonal belt or a distributed loading surface. Kent et al. (2006) previously published juvenile abdominal response data collected using a porcine model. This paper reports on a series of experiments on a 7-year-old pediatric post-mortem human subject (PMHS) undertaken to guide the scaling of existing adult thoracic response data for application to the child and to assess the validity of the porcine abdominal model. The pediatric PMHS exhibited abdominal response similar to the swine, including the degree of rate sensitivity. The upper abdomen of the PMHS was slightly stiffer than the porcine behavior, while the lower abdomen of the PMHS fit within the porcine corridor. Scaling of adult thoracic response data using any of four published techniques did not successfully predict the pediatric behavior. All of the scaling techniques intrinsically reduce the stiffness of the adult response, when in reality the pediatric subject was as stiff as, or slightly more stiff than, published adult corridors. An assessment of age-related changes in thoracic stiffness indicated that for both a CPR patient population and dynamic diagonal belt loading on a PMHS population, the effective stiffness of the chest increases through the fourth decade of life and then decreases, resulting in stiffness values approximately the same for children and for elderly adults. Additional research is needed to elucidate the generality of this finding and to assess its significance for scaling adult data to represent pediatric responses.

The transient relationship between pressure and volume in the pediatric pulmonary system

An accurate understanding of the relationship between pulmonary pressure and volume is required for modeling pulmonary mechanics in a variety of clinical applications. In this study the experimental techniques and mathematical formulations used to characterize viscoelastic materials are applied to characterize transient pulmonary compliance in juvenile swine. Fixed volumes of air were insufflated into 5 swine and held constant for 45 s while the transient decay in tracheal pressure was measured. An analytical model was developed using an optimization scheme that maximized the model fit to the experimental data over the entire time convolution. The initial injected volume was varied to assess the spatial and temporal linearity of the behavior. Model performance was assessed by comparing measured and predicted pressure during insufflations of erratic volume waveforms. It is concluded that the pulmonary impedance of healthy juveniles can be adequately described over a wide volume and frequency range using a relatively simple 5-parameter model that is linear both spatially and temporally.

Viscoelastic response of the thorax under dynamic belt loading

OBJECTIVE

Three postmortem human surrogates (PMHS) were positioned and rigidly mounted through the spine to a tabletop test fixture for the purpose of characterizing thoracic response to diagonal belt loading with well-defined boundary conditions.

METHODS

These PMHS were mounted to a stationary apparatus that supported the spine and shoulders in a configuration comparable to that seen in a 48 km/h automobile sled test at the time of maximum chest deformation. A belt restraint was positioned across the anterior torso with attachments at D-ring and buckle locations based on the geometry of a mid-sized sedan. The belt was attached to a trolley driven by a hydraulic ram linked to a universal test machine. Ramp and hold experiments were conducted at rates of 0.5, 0.9, and 1.2 m/s and hold times of 60 s. Ramp-hold displacement waveforms of up to 20 percent of the chest depth were applied to the chest while the resulting belt loads and spinal reaction loads were recorded. These data were used to identify parameters in a seven-parameter thoracic structural model mathematically analogous to a viscoelastic material model. A final test with 40 percent deflection was performed at the completion of the loading sequence.

RESULTS

Model fits to ramps of different magnitudes indicated that the assumption of temporal linearity was reasonable over the range of inputs in this study. In agreement with previous studies, the spatial (force-deflection) response was only slightly nonlinear, indicating that a fully linear model would be reasonable up to the deflection levels used here.

CONCLUSIONS

Pronounced variability in the instantaneous elastic behavior was observed among the three test subjects, whereas the relaxation behavior exhibited less variability.

The temperature-dependent viscoelasticity of porcine lumbar spine ligaments

STUDY DESIGN

A uniaxial tensile loading study of 13 lumbar porcine ligaments under varying environmental temperature conditions.

OBJECTIVES

To investigate a possible temperature dependence of the material behavior of porcine lumbar anterior longitudinal ligaments.

SUMMARY OF BACKGROUND DATA

Temperature dependence of the mechanical material properties of ligament has not been conclusively established.

METHODS

The anterior longitudinal ligaments (ALLs) from domestic pigs (n = 5) were loaded in tension to 20% strain using a protocol that included fast ramp/hold and sinusoidal tests. These ligaments were tested at temperatures of 37.8 degrees C, 29.4 degrees C, 21.1 degrees C, 12.8 degrees C, and 4.4 degrees C. The temperatures were controlled to within 0.6 degrees C, and ligament hydration was maintained with a humidifier inside the test chamber and by spraying 0.9% saline onto the ligament. A viscoelastic model was used to characterize the force response of the ligaments.

RESULTS

The testing indicated that the ALL has strong temperature dependence. As temperature decreased, the peak forces increased for similar input peak strains and strain rates. The relaxation of the ligaments was similar at each temperature and showed only weak temperature dependence. Predicted behavior using the viscoelastic model compared well with the actual data (R2 values ranging from 0.89 to 0.99). A regression analysis performed on the viscoelastic model coefficients confirmed that relaxation coefficients were only weakly temperature dependent while the instantaneous elastic function coefficients were strongly temperature dependent.

CONCLUSIONS

The experiment demonstrated that the viscoelastic mechanical response of the porcine ligament is dependent on the temperature at which it is tested; the force response of the ligament increased as the temperature decreased. This conclusion also applies to human ligaments owing to material and structural similarity. This result settles a controversy on the temperature dependence of ligament in the available literature. The ligament viscoelastic model shows a significant temperature dependence on the material properties; instantaneous elastic force was clearly temperature dependent while the relaxation response was only weakly temperature dependent. This result suggests that temperature dependence should be considered when testing ligaments and developing material models for in vivo force response, and further suggests that previously published material property values derived from room temperature testing may not adequately represent in vivo response. These findings have clinical relevance in the increased susceptibility of ligamentous injury in the cold and in assessing the mechanical behavior of cold extremities and extremities with limited vascular perfusion such as those of the elderly.

Failure properties of cervical spinal ligaments under fast strain rate deformations

STUDY DESIGN

The failure responses of the anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum were examined in vitro under large strain-rate mechanical loading.

OBJECTIVE

To quantify the failure properties for 3 cervical spinal ligaments at strain rates associated with traumatic events.

SUMMARY OF BACKGROUND DATA

There exists little experimentation literature for fast-rate loading of the cervical spine ligaments. The small amount of available information is framed only in extensive experimental coordinates, and not in the context of strains.

METHODS

Bone-ligament-bone complexes were strained at fast rates, in an incrementally increasing loading protocol using a servohydraulic mechanical test frame. Failure loads and displacements were converted to engineering and true stress and strain values, and compared for the different ligaments (anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum), spinal levels (C3-C4, C5-C6, and C7-T1), and for male versus female specimens.

RESULTS

There were no significant differences in force or true stress for gender or spinal level. There was a significant difference in force and true stress for ligament type. A difference was found between the posterior longitudinal ligament and ligamentum flavum for failure force, and between the ligamentum flavum and both the anterior and posterior longitudinal ligaments for failure true stress. No significant differences were found in true strain for ligament, gender, or spinal level. The mean ligament failure true strain was 0.81. Failure true strains were approximately 57% of the failure engineering strains.

CONCLUSIONS

Once the injury mechanisms of the cervical spine are fully understood, computational models can be employed to understand the potentially traumatic effects of clinical procedures, and mitigate injury in impact, falls, and other high-rate scenarios. The soft tissue failure properties in this study can be used to develop failure tolerances in fast-rate loading scenarios. Failure properties of the anterior and posterior longitudinal ligaments were similar, and the same properties can be used to model both ligaments.

Upper extremity interaction with a helicopter side airbag: injury criteria for dynamic hyperextension of the female elbow joint

This paper describes a three part analysis to characterize the interaction between the female upper extremity and a helicopter cockpit side airbag system and to develop dynamic hyperextension injury criteria for the female elbow joint. Part I involved a series of 10 experiments with an original Army Black Hawk helicopter side airbag. A 5(th) percentile female Hybrid III instrumented upper extremity was used to demonstrate side airbag upper extremity loading. Two out of the 10 tests resulted in high elbow bending moments of 128 Nm and 144 Nm. Part II included dynamic hyperextension tests on 24 female cadaver elbow joints. The energy source was a drop tower utilizing a three-point bending configuration to apply elbow bending moments matching the previously conducted side airbag tests. Post-test necropsy showed that 16 of the 24 elbow joint tests resulted in injuries. Injury severity ranged from minor cartilage damage to more moderate joint dislocations and severe transverse fractures of the distal humerus. Peak elbow bending moments ranged from 42.4 Nm to 146.3 Nm. Peak bending moment proved to be a significant indicator of any elbow injury (p = 0.02) as well as elbow joint dislocation (p = 0.01). Logistic regression analyses were used to develop single and multiple variate injury risk functions. Using peak moment data for the entire test population, a 50% risk of obtaining any elbow injury was found at 56 Nm while a 50% risk of sustaining an elbow joint dislocation was found at 93 Nm for the female population. These results indicate that the peak elbow bending moments achieved in Part I are associated with a greater than 90% risk for elbow injury. Subsequently, the airbag was re-designed in an effort to mitigate this as well as the other upper extremity injury risks. Part III assessed the redesigned side airbag module to ensure injury risks had been reduced prior to implementing the new system. To facilitate this, 12 redesigned side airbag deployments were conducted using the same procedures as Part I. Results indicate that the re-designed side airbag has effectively mitigated elbow injury risks induced by the original side airbag design. It is anticipated that this study will provide researchers with additional injury criteria for assessing upper extremity injury risk caused by both military and automotive side airbag deployments.

Muscle tetanus and loading condition effects on the elastic and viscous characteristics of the thorax

Thoracic deformation under an applied load is an established indicator of injury risk, but the force required to achieve an injurious level of deformation currently is not understood adequately. This article evaluates how two potentially important factors, loading condition and muscle tensing, affect the structural response of the dynamically loaded thorax. Structural models of two human cadaver thoraxes and two porcine thoraxes were used to quantify the effects. The human cadavers, which represent anthropometric extremes, were subjected to anterior loading from (1) a 5.1-cm-wide belt oriented diagonally (i.e., seatbelt-like loading), (2) a 15.2-cm-diameter rigid hub, and (3) a 20.3-cm-wide belt oriented laterally (i.e., a distributed load). A structural model having the mathematical formulation of a quasilinear viscoelastic material model was used to model the elastic and viscous response, with ramp-hold tests used to determine the model coefficients. The effect of thoracic musculature was assessed using similar ramp-hold tests on the porcine subjects, each with and without forced muscle contraction. Even maximally contracted thoracic musculature is shown to have a minimal effect on the response, with similar elastic and viscous characteristics exhibited by each subject regardless of muscle tone. The elastic response is shown to be approximately a factor of three stiffer for diagonal belt loading and for this distributed loading condition than for the hub loading, indicating that the response is influenced most by the particular anatomical structures that are engaged and, secondarily, by the area of load application. Specifically, shoulder involvement is shown to have a strong influence. The force relaxation is found to be pronounced, but insensitive to the loading condition, with long-time force relaxation coefficients (G( infinity )) in the range of 0.1 to 0.3. The findings of this study provide restraint-specific guidelines for the force-deflection characteristics of both physical and computational thoracic models.

The development, validation and application of a finite element upper extremity model subjected to air bag loading

Both frontal and side air bags can inflict injuries to the upper extremities in cases where the limb is close to the air bag module at the time of impact. Current dummy limbs show qualitatively correct kinematics under air bag loading, but they lack biofidelity in long bone bending and fracture. Thus, an effective research tool is needed to investigate the injury mechanisms involved in air bag loading and to judge the improvements of new air bag designs. The objective of this study is to create an efficient numerical model that exhibits both correct global kinematics as well as localized tissue deformation and initiation of fracture under various impact conditions. The development of the model includes the creation of a sufficiently accurate finite element mesh, the adaptation of material properties from literature into constitutive models and the definition of kinematic constraints at articular joint locations. In order to make the model applicable for full-scale simulations, it was coupled with a computationally efficient human model. The model was validated against available cadaver experiments, including static and dynamic three-point-bending tests to the arm and forearm, as well as frontal air bag to forearm impact tests. The sensitivity of the model to changes in air bag properties and upper limb orientation are demonstrated by performing parametric studies. It is shown that the risk of forearm fracture increases substantially with proximity to the deploying frontal air bag and air bag aggressiveness, which corresponds to experimental findings. However, it is shown that increasing the forearm supination angle is protective for the occurrence of forearm fracture. In conclusion, the developed model proves to be a useful research tool to investigate trends in injury severity as a result of a changing frontal air bag to upper extremity loading environment.